HI
I have endless experience running the OS 61 sf engine with no pump on 15 % morgan omega fuel
my first thought is the diaphragm in the pump has dried up and gone bad as they will over time-,try to get a stock , no pump back plate if you can-,no parts are available for the pump if you need a pump you can run an external perry pump off the stock back plate with a pressure fitting
the bearings in these engines will last for many years using the fuel i suggested if you run the engine correctly-not lean ! the engine is extremely user friendly
the air in the fuel line appears to be from the pump so my recommendation above
be very sure that all gasket surfaces are clean an no air is getting into the crankcase anywhere
the engine will run hot if air is getting in
do not attempt to adjust the needle valves when the engine is hot it must cool completely down before each run then start tune over
i run a 12/6 or a 13/6 apc on my engines i have three and been running a 61 SF for 20 years that i s when they first came out
if you run a glo engine lean on a full syn fuel you will fry the engine -, castor oil breaks down over 100 degrees higher then of that of a syn fuel and bond to the metal parts at this time just when you need the protection
running a glo engine lean at some time is a given although never intended it happens to the best of us and must be identified as quickly as is possible and corrected
just my two cents
BEST REGARDS TONY